He resigned because he'd already had enough trouble as president. Mainly the Watergate scandal. His plan was for him to step down and if he was arrested the vice president which would then be the vice president would pardon him. Also, if he didn't resign he'd likely be impeached anyway.
The plague help lead to an economic collapse in the third century is that it contributed to a sharp decline in the supply of labor , hurting the Roman economy
A system of government by the whole population or all the eligible members of a state, typically through elected representatives.
